I don`t know if there is a thread for that (sorry if there is, I haven`t look) But here is a humble list of the best musician (I know) on the usual (rock) instruments.

Guitar : Paco De Lucia
Bass : Wooten, Manring
Drum : Dennis Chambers, Jack DeJohnette

I didn`t put Vocals how can you compare Freddy Mercury to Pavarotti, Celine Dion, Mariah Carey or other Domingo/Carreras of this world... I can`t.

Same with Keyboards : Keith Emerson, Yngwie Malmstein keyboardist, or any player who can play Rachmaninov are more than worthy contenders...

Another debate altogether.

I`m presuming no-one here plays bass...
Geddy Lee and Claypool rock, but are no way the best.

Drums - Neal Peart
Guitar - Jimi Hendrix
Bass - Jaco Pastorius (undoubtedly the best bassist ever)
vocals - (varies in style..id go for Chris Cornell)

This would be a dream one for me..

Drums - Neal Peart
Bass - Les Claypool
Guitar - Steve Vai
Vocals - Ozzy Osbourne

ok u guys are prolly gonna say wtf?

but here goes nothin

singer: deryck whibley,this guy has an amazing voice, u guys are just gonna say he sucks because hes in Sum 41
guitarist: JEFF HEALEY NO QUESTION, jeff is the most talented guitarist EVER, and hes blind to boot. hes not very well known however but critics say he is better than hendrix, who is my second pick
bassist: Tim Commerford of Rage Against the Machine/Audioslave, hes not the most talented but he is pretty talented and it shows more when he was in rage.
drummer: neil peart no doubt

Vocal-Steven Tyler
Guitar-Carlos Santana, Eric Clapton, Vn Hallen
Bass-John Entwistle
Drums-Ringo Starr

SINGER....mercury hell yeah
BASS...John Entwhistle(the who)
GUITARIST...it`s quite close between...hendrix...page...gilmore...and clatpon...at the moment...i have to say david gilmore
DRUMMER...i am a drummer and all u drummers out there should all say John henry bonham...cause of moby dick...neilr peart could never beat bonham`s live version on "How The West Was Won"...i also like mike portnoy a lot...keith moon...and ginger baker...but overall...BONHAM beats all

the end

Vocals: Axl Rose, Joey Tempest, Steven Tyler, Anthony Kiedis
Guitar: Slash, Joe Perry, Izzy Stradlin
Drums: Steven Adler
Bass: Flea, "Duff" McKagan

Guitar - Hendrix or Steve Vai
Bass- Geddy Lee
Drums - Old: i gotta give this one to Neil Pert, yyz is awesome
New: Travis Barker is ill and is the best mordern rock drummer by far

joey jordison from slipknot and neil peart

Best Guitarist are as follows, in order
Jimmy Page of Led Zeppelin
Santana
Jimi Hendrix
Eric Clapton
Tommy Moralo of Rage Against the Machine

Guitarist: Michael Angelo Batio, Yngwie Malmsteen, Paul Gilbert, Joe Satriani, Steve Vai, John Petrucci, Jimi Hendrix, James Hetfield, Kirk Hammett, Eddie Van Halen, Jason Becker, + more (The first 3 are my fav)

Drummer: Bobby Rock, Joey Jordison, Jeff Campitelli (However you spell it), plus more.

Bassist: Billy Sheehan

Vocals: Jim Gillete, Hillary duff!

VOCALS-Jesse Michaels
Guitar- Tim Armstrong
Bass- Matt Freeman
Drums- Dave Mello

the best rock-guitarist ever was Jimi Hendrix no question.
He could do all the rock styles, play the guitar with his teeth, play the guitar with one hand behind his back, play the guitar on his head. He also created a unique rock style that no one ever had been able to comprehend.

Vocals: Jon Anderson, Peter Gabriel
Guitar: Robert Fripp, Steve Howe, Andy Latimer
BAss: Chris Squire
Drummer: Bill Bruford, Carl Palmer
Kyboards: Rick Wakeman

Okay, here`s who I think is the best in each field:

Vocals - Freddie Mercury, no question. Definitely the best vocalist rock music gave us
Guitar - Eric Clapton, Randy Rhoads, Jimi Hendrix, Jimmy Page, Pete Townshend, Angus Young, George Harrison.
Bass - Jaco Pastorius (the best bassist ever to live), Geddy Lee, John Entwistle, Jack Bruce, John Paul Jones, Paul McCartney, Flea, Les Claypool, Marcus Miller, Nathan East.I`m a bass player so I`ve studied these guys
Drums - Neil Peart, Travis Barker, Keith Moon. these three stand out the most to me.
Keys - Elton John, Billy Joel, Stevie Wonder.